Power semiconductor device

ABSTRACT

A semiconductor device includes a vertical power semiconductor chip including a semiconductor layer. A first terminal is at a first side of the semiconductor layer and a second terminal is at a second side of the semiconductor layer opposite the first side along a first direction. A drift zone is within the semiconductor layer between the first terminal and the second terminal. The drift zone has, in a central part, a compressive stress of at least 100 MPa along a second direction perpendicular to the first direction. The central part extends from 40% to 60% of an overall extension of the drift zone along the first direction and into a depth of the semiconductor layer of at least 10 μm with respect to at least one of the first side and the second side of the semiconductor layer.

BACKGROUND

Semiconductor device manufacturers constantly strive to increase theperformance of their products while decreasing cost of manufacture. Inpower semiconductor devices a trade-off between a withstand voltage andan on-state resistance has to be met. Therefore, it is desirable toreduce the on-state resistance in a state in which the withstand voltageis secured.

SUMMARY

According to an embodiment of a semiconductor device, the semiconductordevice includes a vertical power semiconductor chip having asemiconductor layer. The semiconductor device further includes a firstterminal at a first side of the semiconductor layer and a secondterminal at a second side of the semiconductor layer opposite the firstside along a first direction. The semiconductor device further includesa drift zone within the semiconductor layer between the first terminaland the second terminal. The drift zone has, in a central part, acompressive stress of at least 100 MPa along a second directionperpendicular to the first direction, the second part extending from 40%to 60% of an overall extension of the drift zone along the firstdirection and into a depth of the semiconductor layer of at least 10 μmwith respect to at least one of the first side and the second side ofthe semiconductor layer.

According to another embodiment of a vertical power semiconductortransistor, the vertical power semiconductor transistor includes a firstterminal at a first side of a semiconductor layer and a second terminalat a second side of the semiconductor layer opposite the first sidealong a first direction. The vertical power semiconductor transistorfurther includes a drift zone within the semiconductor layer between thefirst terminal and the second terminal. The drift zone has, in a centralpart, a compressive stress of at least 100 MPa along a second directionperpendicular to the first direction, the central part extending from40% to 60% of an overall extension of the drift zone along the firstdirection and into a depth of the semiconductor layer of at least 10 μmwith respect to at least one of the first side and the second side ofthe semiconductor layer. The vertical power semiconductor transistorfurther includes at least one stressor layer arranged above at least oneof the first side and the second side of the semiconductor layer. The atleast one stressor layer is configured to induce a compressive stresswithin the drift zone. The vertical power semiconductor transistor isconfigured to be operated at a voltage equal to or larger than 100 V.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The elements of the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding similarparts. The features of the various illustrated embodiments can becombined unless they exclude each other. Embodiments are depicted in thedrawings and are detailed in the description which follows.

FIG. 1 illustrates a schematic cross-section of one embodiment of avertical power semiconductor device including a compressive stresswithin a drift zone of a semiconductor layer.

FIG. 2 illustrates a schematic cross-section of one embodiment of avertical power semiconductor device including a stressor layer arrangedbetween a drift zone of a semiconductor layer and a semiconductorsubstrate.

FIG. 3 illustrates a schematic cross-section of one embodiment of avertical power semiconductor device including a metal stressor layeradjoining to a drift zone of a semiconductor layer.

FIG. 4 illustrates a schematic cross-section of one embodiment of avertical power semiconductor device including a stressor layer arrangedat a front side of the device.

FIG. 5 illustrates a schematic cross-section of one embodiment of apower semiconductor device mounted on a DCB (Direct Copper Bond)substrate and including a compressive stress within a drift zone of asemiconductor layer.

FIG. 6 is a schematic cross-section of one embodiment of a semiconductordevice including a vertical power semiconductor chip mounted on anelectrically conducting carrier.

FIG. 7 is a cross-sectional view of one detailed embodiment inaccordance with the embodiment illustrated in FIG. 6.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, an in which is shown byway of illustration specific embodiments in which the invention may bepracticed. In this regard, directional terminology, such as “top”,“bottom”, “front”, “back”, “leading”, “trailing”, etc., is used withreference to the orientation of the figure(s) being described. Becausecomponents of the embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, thereof, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

A number of embodiments will be explained below. In this case, identicalstructural features are identified by identical or similar referencesymbols in the Figures. In the context of the present description,“lateral” or “lateral direction” should be understood to mean adirection or extent that runs generally parallel to the lateral extentof a semiconductor material or semiconductor carrier. The lateraldirection thus extends generally parallel to these surfaces or sides. Incontrast thereto, the term “vertical” or “vertical direction” isunderstood to mean a direction that runs generally perpendicular tothese surfaces or sides and thus to the lateral direction. The verticaldirection therefore runs in the thickness direction of the semiconductormaterial or semiconductor carrier.

As employed in this specification, the terms “coupled” and/or“electrically coupled” are not meant to mean that the elements must bedirectly coupled together—intervening elements may be provided betweenthe “coupled” or “electrically coupled” elements.

Semiconductor devices including power semiconductor chips are describedbelow. The power semiconductor chips may be of different types, may bemanufactured by different technologies and may include, for example,integrated electrical, electro-optical or electro-mechanical circuits orpassives. The power semiconductor chips need not be manufactured fromspecific semiconductor material, for example, Si, SiC, SiGe, GaAs, and,furthermore, may contain inorganic and/or organic materials that are notsemiconductors, such as, for example, discrete passives, antennas,insulators, plastics or metals. Furthermore, the devices described belowmay include further integrated circuits to control the power integratedcircuits of the power semiconductor chips.

The power semiconductor chips may comprise power MOSFETs (Metal OxideSemiconductor Field Effect Transistors), DMOSFETs (Double-diffusedMOSFET), IGBTs (Insulated Gate Bipolar Transistors), JFETs (JunctionGate Field Effect Transistors), power bipolar transistors or powerdiodes. More specifically, the power semiconductor chips having avertical structure are involved, that is to say that the powersemiconductor chips, more specifically vertical power semiconductorchips, are fabricated in such a way that electric currents can flow in adirection perpendicular to the main faces of the power semiconductorchips.

A power semiconductor chip having a vertical structure, i.e., verticalpower semiconductor chip, may have terminals such as contact pads on itstwo main faces, that is to say on its top side and bottom side, or, inother words, on its front side and rear side. By way of example, thesource electrode and the gate electrode of a power MOSFET may besituated on one main face, while the drain electrode of the power MOSFETmay be arranged on the other main face. The contact pads may be made ofaluminum, copper or any other suitable material. One or more metallayers may be applied to the contact pads of the power semiconductorchips. The metal layers may, for example, be made of titanium, nickelvanadium, gold, silver, copper, palladium, platinum, nickel, chromium orany other suitable material. The metal layers need not be homogenous ormanufactured from just one material, that is to say various compositionsand concentrations of the materials contained in the metal layers arepossible.

The power semiconductor chip may be made of a bulk semiconductor such asa semiconductor substrate and a semiconductor layer such as an epitaxiallayer generated on the bulk semiconductor. The epitaxial layer may havea thickness greater than or less than the thickness of the bulksemiconductor and may include a drift zone. As an example, the epitaxiallayer may have a thickness of equal or greater than 20 μm, 30 μm, 40 μmor 50 μm. Typically, the greater the thickness of the epitaxial layer,the higher is the operating voltage of the power semiconductor chip.

An encapsulation material may at least partially cover the verticalpower semiconductor chip to form an encapsulation body. Theencapsulation material may be based on a polymer material, that is itmay include a basis material (also referred to as a matrix material inthe following) made of any appropriate duroplastic, thermoplastic orthermosetting material or laminate (prepreg). In particular, a matrixmaterial based on epoxy resin may be used. The matrix material may embeda filler material, for instance SiO₂, Al₂O₃ or AlN particles to adjustphysical properties of the encapsulation body such as, e.g., the elasticmodulus or the CTE (coefficient of thermal expansion).

After its deposition, the encapsulation material may only be partiallyhardened and may then be cured and/or completely hardened by theapplication of energy (e.g., heat, UV light, etc.) to form the solidencapsulation body. Various techniques may be employed to form theencapsulation body by the encapsulation material, for example,compression molding, transfer molding, injection molding, powdermolding, liquid molding, dispensing or laminating. Heat and/or pressuremay be used to apply the encapsulation material.

FIG. 1 illustrates a cross-section of one embodiment of a vertical powersemiconductor device 100. The vertical semiconductor power device 100includes a semiconductor layer 105, e.g., an epitaxial semiconductorlayer. A drift zone 110 constitutes a part of the semiconductor layer105. For illustration purposes, the drift zone 110 illustrated in FIG. 1extends from a first surface 115 at a first side such as a top side orfront side of the semiconductor layer 105 to a second surface 120 at asecond side such as a bottom side or rear side of the semiconductorlayer 105. The overall extension of the drift zone 110 from the firstsurface 115 to the second surface 120 along a first direction y, inparticular a vertical direction, is denoted by d₁. The term “overallextension of the drift zone along the first direction” is meant to meanthat extension along the first direction from where charge carriersenter the drift zone, e.g., at an end of a channel, to the locationwhere these charge carriers enter a terminal, e.g., a metal contact suchas a drain contact or a semiconductor region having a higher dopantconcentration than the drift zone such as a highly doped semiconductorsubstrate.

A first terminal 125 is arranged at the first side and a second terminal130 is arranged at the second side. An electrical current within thevertical power semiconductor device 100 flows between the first terminal125 and the second terminal 130 along the first direction y. In oneembodiment, the first terminal 125 is a source terminal including one ora plurality of conductive elements such as metal areas and/or contactplugs and the second terminal 130 is a drain terminal and may includeone or a plurality of metal layers adjoining to a semiconductor materialsuch as a semiconductor substrate or the semiconductor layer 105. Inanother embodiment, the first terminal 125 is a cathode of a power diodeand the second terminal 130 is an anode of the power diode.

It is to be noted that FIG. 1 is a simplified illustration of the firstterminal 125, the drift zone 110 and the second terminal 130. Althoughthe sequence of these elements along the first direction y is in theorder of the first terminal 125, the drift zone 110 and the secondterminal 130, a shape of the illustrated elements may differ from thesimplified illustration of FIG. 1 and intervening elements may bearranged between the semiconductor layer 105 and the first terminal 125as well as between the semiconductor layer 105 and the second terminal130, e.g., a semiconductor substrate may be arranged between thesemiconductor layer 105 and the second terminal 130. In other words,further elements may be arranged between the first surface 115 of thesemiconductor layer 105 and the first terminal 125 at the first side andbetween the second surface 120 of the semiconductor layer 105 and thesecond terminal 130 at the second side.

In a central part of the drift zone 110 that extends from 40% of theoverall extension d₁ of the drift zone 110 along the first direction y,i.e., from y₀, to 60% of the overall extension d₁ of the drift zone 110along the first direction y, i.e., to y₁, a compressive stress σ amountsto at least 100 MPa along a second direction x that is perpendicular tothe direction of current flow, i.e., perpendicular to the firstdirection y. The compressive stress σ in the central part of the driftzone 110 is illustrated by opposing arrows and leads to an improvementof the carrier mobility and thus to an improved on-state resistance.

The central part extends into a depth of the semiconductor layer 105 ofat least 10 μm with respect to at least one of the first surface 115 ofthe semiconductor layer 105 at the first side and the second surface 120at the second side of the semiconductor layer 105. In other words, inthe embodiment illustrated in FIG. 1 a distance between a vertical levelcoinciding with the first surface 115 and y₀, amounts to 0.4×d of atleast 10 μm. Similarly, a distance between the level y₁ and the secondsurface 120 at the second side amounts to 0.4×d of at least 10 μm.

The compressive stress σ in the central part of the drift zone 110 isinduced by a stressor layer (not illustrated in FIG. 1). In oneembodiment, the stressor layer constitutes part of the first terminal125 and/or the second terminal 130. In another embodiment, the stressorlayer constitutes part of or equals other elements not illustrated inFIG. 1, e.g., elements arranged below or above the first terminal 125and/or elements arranged below or above the second terminal 130.

In the embodiments illustrated in FIGS. 2 to 7, examples for thestressor layer configured to induce the compressive stress within thecentral part of the drift zone are given.

FIG. 2 illustrates a cross-section of one embodiment of a vertical powersemiconductor device 200. Similar to the vertical power semiconductordevice 100 of the embodiment illustrated in FIG. 1, the vertical powersemiconductor device 200 includes a semiconductor layer 205 and a firstterminal 225 at a first side of the semiconductor layer 205 as well as asecond terminal 230 at a second side of the semiconductor layer 205. Acentral part of a drift zone 210 of the semiconductor layer 205, whichextends as specified in the embodiment illustrated in FIG. 1, includes acompressive stress σ of at least 100 MPa along a second direction xperpendicular to the direction of current flow, i.e., perpendicular to afirst direction y.

The compressive stress σ is induced by a stressor layer 240 adjoining toa second surface 220 of the semiconductor layer 205 at the second side.Opposite to the second surface 220 of the stressor layer 240, thestressor layer 240 adjoins to a substrate 245. In one embodiment, thesubstrate 245 is a semiconductor substrate. The stressor layer 240 mayinclude one or a plurality of conducting materials and provides anelectrically conducting path between the drift zone 210 and thesubstrate 245 along the first direction y. According to one embodiment,the stressor layer 240 is a continuous layer. According to anotherembodiment, the stressor layer 240 is a patterned layer includingseparate stressor regions such as stressor islands, for example.

In the embodiment illustrated in FIG. 2, the substrate 245 and thestressor layer 240 are arranged between the second surface 220 of thesemiconductor layer 205 and the second terminal 230 at the second side.The second terminal 230, the substrate 245, the stressor layer 240, thesemiconductor layer 205 including the drift zone 210, and the firstterminal 225 constitute part of a vertical power semiconductor chip 255.

The vertical power semiconductor device 200 includes a carrier plate 250such as a leadframe and the vertical power semiconductor chip 255mounted on the carrier plate 250 via the second terminal 230.

FIG. 3 illustrates a cross-section of another embodiment of a verticalpower semiconductor device 300. Similar to the vertical powersemiconductor device 100 of the embodiment illustrated in FIG. 1, thevertical power semiconductor device 300 includes a semiconductor layer305 and a first terminal 325 at a first side of the semiconductor layer305 as well as a second terminal 330 at a second side of thesemiconductor layer 305. A central part of a drift zone 310 of thesemiconductor layer 305, which extends as specified in the embodimentillustrated in FIG. 1, includes a compressive stress σ of at least 100MPa along a second direction x perpendicular to the direction of currentflow, i.e., perpendicular to a first direction y.

The second terminal 330 equals or constitutes part of the stressor layer340 and adjoins a second surface 320 of the semiconductor layer 305.

The stressor layer 340 can include at least one metal or metal alloy.According to one embodiment, the at least one metal or metal alloy isformed on the second surface 320 of the semiconductor layer 305 by anappropriate method such as sputtering, deposition, evaporation, etc.According to another embodiment, the stressor layer includes at leastone metal foil on the second surface 320 of the semiconductor layer 305.The metal foil may be a Cu foil or may include Cu.

A thickness of the stressor layer 340 is appropriately chosen so as toinduce the compressive stress σ of at least 100 MPa in the central partof the drift zone 310. The thickness may of the stressor layer 340 be ina range of 50 μm to 1 mm, or 100 μm to 1 mm, or 200 μm to 1 mm.

The second terminal 330, the semiconductor layer 305 including the driftzone 310, and the first terminal 325 constitute part of a vertical powersemiconductor chip 355.

The vertical power semiconductor device 300 includes a carrier plate 350such as a leadframe and the vertical power semiconductor chip 355mounted on the carrier plate 350 via the second terminal 330.

FIG. 4 illustrates a cross-section of yet another embodiment of avertical power semiconductor device 400. Similar to the vertical powersemiconductor device 100 of the embodiment illustrated in FIG. 1, thevertical power semiconductor device 400 includes a semiconductor layer405 and a first terminal 425 at a first side of the semiconductor layer405 as well as a second terminal 430 at a second side of the firstsemiconductor layer 405. A central part of a drift zone 410 of thesemiconductor layer 405, which extends as specified in the embodimentillustrated in FIG. 1, includes a compressive stress σ of at least 100MPa along a second direction x perpendicular to the direction of currentflow, i.e., perpendicular to a first direction y.

The semiconductor layer 405 is formed on one surface of a semiconductorsubstrate 445 such as a semiconductor wafer, e.g., by epitaxial growth.In one embodiment, the semiconductor substrate 445 is a highly dopedsemiconductor substrate. The second terminal 430 adjoins the othersurface of the semiconductor substrate 445.

A stressor layer 440 is arranged above the first terminal 425. Accordingto other embodiments, the stressor layer 440 may equal or constitutepart of the first terminal 425.

A material and thickness of the stressor layer 440 is appropriatelychosen so as to induce the compressive stress σ of at least 100 MPa inthe central part of the drift zone 410. The central part of the driftzone 410 refers to the part as described with regard to the embodimentillustrated in FIG. 1. According to one embodiment, the stressor layer440 includes one or a plurality of metals or metal alloys such as metallayers, e.g., a Cu layer, metal alloy layers, metal foils, e.g., a Cufoil, or any combination thereof. A thickness of the stressor layer 440may be in a range of 50 μm to 1 mm, or 100 μm to 1 mm, or 200 μm to 1mm. The stressor layer 440 may also, in addition to metal or metal alloyor alternatively to metal or metal alloy, include at least one of oxide,silicide, polysilicon, organic coating.

The second terminal 430, the semiconductor substrate 445, thesemiconductor layer 405 including the drift zone 410, the first terminal425 and the stressor layer 440 constitute part of a vertical powersemiconductor chip 455.

The vertical power semiconductor device 400 includes a carrier plate 450such as a leadframe and the vertical power semiconductor chip 455mounted on the carrier plate 450 via the second terminal 430.

FIG. 5 illustrates a cross-section of yet another embodiment of avertical power semiconductor device 500. Similar to the vertical powersemiconductor device 100 of the embodiment illustrated in FIG. 1, thevertical power semiconductor device 500 includes a semiconductor layer505 and a first terminal 525 at a first side of the semiconductor layer505 as well as a second terminal 530 at a second side of thesemiconductor layer 505. A central part of a drift zone 510 of thesemiconductor layer 505, which extends a specified in the embodimentillustrated in FIG. 1, includes a compressive stress σ of at least 100MPa along a second direction x perpendicular to the direction of currentflow, i.e., perpendicular to a first direction y.

The second terminal 530, the semiconductor layer 505 including the driftzone 510, and the first terminal 525 constitute part of a vertical powersemiconductor chip 555.

The vertical power semiconductor chip 555 is mounted on a DCB substrate560. The DCB substrate 560 includes a first Cu layer 561 and a second Culayer 562 arranged on opposite surfaces of an insulating plate 563 suchas a ceramic plate. The DCB substrate 560 is arranged on a bottom plate565 such as a Cu plate via the second Cu layer 562 and an adhesive layer566 such as a solder layer.

The compressive stress in the central part of the drift zone 510 isinduced by a stressor layer (not illustrated in FIG. 5). Examples forthe stressor are given in the embodiments illustrated in FIGS. 2 to 4.The compressive stress σ in the central part of the drift zone 510 mayalso be induced by mounting and connecting the vertical powersemiconductor chip 555 in a power package, e.g., SuperSO8, via a stiffadhesive layer, e.g., diffusion solder layer such as AuSn, AgSn, CuSn,Sn, AuSi, AuGe, AgIn and connecting the power package to the DCBsubstrate 560 via a connecting layer such as a diffusion solder layer.The stiff adhesive layer between the vertical power semiconductor chip555 and the power package constitutes part of the stressor layer.

FIG. 6 schematically illustrates one embodiment of a power semiconductordevice 600 in cross-section. A power semiconductor chip 655 of the powersemiconductor device 600 has one or more first contact pads 611 arrangedon a first main face 612 and one or more second contact pads 613arranged on a second main face 614 opposite the first main face 612.Further, the power semiconductor device 600 includes a carrier plate 650on which the power semiconductor chip 655 is mounted.

The power semiconductor chip 655 is a vertical device, i.e., electriccurrents can flow in a direction perpendicular to the main faces 612,614 of the power semiconductor chip 655. In one embodiment, the powersemiconductor chip 655 is a power transistor and the first contactpad(s) 611 may form a source terminal and the second contact pad(s) 613may form a drain terminal. In this embodiment, typically, a gateterminal (not shown) is arranged on the first main face. In otherembodiments, the power semiconductor chip 655 may be a power diode, andthe first contact pad(s) 611 may, e.g., form an anode terminal and thesecond contact pad(s) 613 may form a cathode terminal of the power diodeor vice versa.

More specifically, the power semiconductor chip 655 may include anepitaxial layer 605 arranged, e.g., on a bulk semiconductor 645. Aperson skilled in the art is well aware of multiple semiconductorprocessing techniques to produce such a structure. Briefly, theepitaxial layer 605 may be fabricated on a wafer made of semiconductormaterial. The upper surface of the wafer may correspond to the uppersurface of the bulk semiconductor 645 in FIG. 6. The wafer, for instancea silicon wafer, may be doped by suitable impurity atoms (dopants) toenhance the electrical conductivity of the semiconductor material of thebulk semiconductor 645. By way of example, the wafer may be doped toobtain a n⁺ doped bulk silicon of high electrical conductivity.

Still during frontend wafer processing, the epitaxial layer 605 may begenerated on the upper surface of the wafer. All epitaxial techniquesknown to a person skilled in the art may be used, e.g., MBE (molecularbeam epitaxy), LPE (liquid phase epitaxy), etc. The epitaxial layer 605may contain a sequence of p-n junctions to form the active semiconductorregion of the power device 600.

The first contact pad(s) 611 are formed on an upper surface of theepitaxial layer 605. This step may still be carried out during waferprocessing, that is during frontend processing. In other embodiments,the first contact pad(s) 611 may be formed after separation of the waferinto multiple power semiconductor chips 655.

Similar to the first contact pad(s) 611, the second contact pad(s) 613are formed either during wafer processing on the intact wafer or on thepower semiconductor chips 655 singularized from the wafer.

Integrated power circuits and possibly further integrated circuits canbe electrically accessed via the contact pads 611, 613. The contact pads611, 613 may be made of a metal, for example, aluminum or copper, andmay have any desired shape and size.

The power semiconductor chip 655 may be mounted on the upper surface ofthe carrier plate 650. A connecting layer 617 of solder material, e.g.,a diffusion solder material comprising e.g., AuSn and/or other metalmaterials may be used to electrically connect and mechanically securethe second contact pad(s) 613 to the carrier plate 650.

The carrier plate 650 may be of various types. In one embodiment thecarrier plate 650 may be a patterned metal sheet or plate, e.g., aleadframe. The carrier plate 650 may have metal plate regions separatedfrom each other by spacings. In another embodiment the carrier plate 650may be a continuous, unpatterned metal plate or sheet. The carrier plate650 may be produced by a stamping and/or milling process. The metal ofwhich the carrier plate is made may, e.g., comprise one or more metalsof the group of copper, aluminum, nickel, gold or any alloy based on oneor more of these metals. The carrier plate 650 (e.g., a leadframe) maybe made of one single bulk metal layer or a multi metal layer structure.The carrier plate 650 may serve as a heat sink for dissipating the heatgenerated in the power semiconductor chip 655.

A compressive stress σ of at least 100 MPa in a central part of a driftzone 610 within the epitaxial layer 605 can be set by dimensions andmaterial properties of one or numerous stressor layers such as the firstand/or second contact pads 611, 613. A definition of the central part isgiven in the embodiment associated with FIG. 1.

FIG. 7 is a cross-sectional view of a more detailed, exemplaryembodiment in accordance with the embodiment shown in FIG. 6. Thesemiconductor device 700 shown in FIG. 7 is a power MOSFET having anoperating voltage of 600 V. A carrier plate 750 is a leadframe made ofcopper having, for instance, a thickness of 1.3 mm. In general, thethickness may be equal or greater than 1.0 mm or even 2.0 mm. Aconnecting layer 717 is made of an AuSn diffusion solder and has athickness of 1.2 μm. A second contact pad 713 may be a Cu layer or Cufoil having a thickness of more than 100 μm, more than 200 μm, more than300 μm or more than 400 μm.

A bulk semiconductor 745 may be made of n⁺ doped silicon and may have athickness in a range of 1 μm to 100 μm, in particular 10 μm to 50 μm.The bulk semiconductor 745 may behave essentially as a metal. Thedegenerate semiconductor-metal junction between the bulk semiconductor745 and the second contact pad 713 also has a high electricalconductivity. The bulk semiconductor 745 and the second contact pad 713represent a drain terminal of the power MOSFET.

An epitaxial layer 705 represents the active region of a powersemiconductor chip 755. In this embodiment, the epitaxial layer 705 hasa thickness of 52.5 μm, allowing for an operating voltage of about 600V. The epitaxial layer 705 includes a series of p-n. In general, theepitaxial layer 705 may have a thickness of equal or greater than 20 μm,in particular, equal or greater than 30 μm, still, in particular, equalor greater 40 μm or, in particular, equal or greater than 50 μm. As arule of thumb, each 100 V of operating voltage require about 9 μm ofsilicon epitaxial layer thickness. Thus, the power semiconductor chip755 may have an operating voltage of equal or greater than 200 V, inparticular equal or greater than 300 V, still in particular equal orgreater than 400 V or in particular equal or greater than 500 V.

A contact zone 741 is provided within the epitaxial layer 705. Thecontact zone 741 is electrically connected to the p-n junctions of theepitaxial layer 705 to form a source contact thereof.

The contact zone 741 is covered by a first insulating layer 742 such as,e.g., an oxide layer, which is provided on top of the epitaxial layer705. The insulating layer 742 is referred to as EOX in FIG. 7 and isconfigured to have openings through which the contact zone 741 may beaccessed.

A first patterned metal layer 743 is arranged on the insulating layer742. The first patterned metal layer 743 may serve to provide anelectrical functionality such as, e.g., electrostatic shielding of thepower MOSFET. Further, additional patterned metal layers not illustratedin the sectional view of FIG. 7 may be arranged in the vicinity of anupper face of the epitaxial layer 705, e.g., patterned metal layers toprovide for additional connectivity and/or signal routing such as, e.g.a patterned metal layer to form a gate contact.

A second patterned insulating layer 744 such as, e.g., an oxide layermay be arranged on the first patterned metal layer 743. The secondinsulating layer 744 is referred to as ZwOX in FIG. 7 and includesopenings through which the contact zone 741 is electrically contacted bya second metal layer 749. The second metal layer 749 may constitute thesource terminal of the power MOSFET. Further, a polymer layer 746 may beprovided to cover the second metal layer 749. A polymer material such asparylene, photoresist material, imide, epoxy, duroplast or silicone maybe used.

By way of example, the layers 742, 743, 744 and 746 may have thefollowing dimensions in thickness. The first insulating layer 742 mayhave a thickness of 2.4 μm, the second insulating layer 744 may have athickness of 1.5 μm, the first patterned metal layer 743 may have athickness of 5.0 μm and the polymer layer 746 may have a thickness of6.0 μm. It is to be noted that the dimensions, materials and theprovision of these layers are exemplary and are subject to variations inaccordance with the needs of the semiconductor design.

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper” and the like are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures.

Further, terms such as “first”, “second”, and the like, are also used todescribe various elements, regions, sections, etc. and are also notintended to be limiting. Like terms refer to like elements throughoutthe description.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

It is to be understood that the features of the various embodimentsdescribed herein may be combined with each other, unless specificallynoted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A semiconductor device, comprising: a verticalpower semiconductor chip including a semiconductor layer; a firstterminal at a first side of the semiconductor layer; a second terminalat a second side of the semiconductor layer opposite the first sidealong a first direction; and a drift zone within the semiconductor layerbetween the first terminal and the second terminal, the drift zonehaving, in a central part, a compressive stress of at least 100 MPaalong a second direction perpendicular to the first direction, thecentral part extending from 40% to 60% of an overall extension of thedrift zone along the first direction and into a depth of thesemiconductor layer of at least 10 μm with respect to at least one ofthe first side and the second side of the semiconductor layer; at leastone stressor layer adjoining the semiconductor layer, the at least onestressor layer configured to induce the compressive stress within thedrift zone.
 2. The semiconductor device of claim 1, wherein the at leastone stressor layer includes at least one of a metal, a metal silicide, adoped silicon, an oxide, and an oxide filled in a trench.
 3. Thesemiconductor device of claim 1, further comprising a semiconductorsubstrate adjoining the at least one stressor layer, the at least onestressor layer arranged between the semiconductor layer and thesemiconductor substrate.
 4. The semiconductor device of claim 3, whereinthe at least one stressor layer is patterned with a plurality ofseparate stressor regions.
 5. The semiconductor device of claim 3,further comprising an electrically conducting carrier attached to thesemiconductor substrate, wherein a distance between the electricallyconducting carrier and the semiconductor layer is larger than 50 μm. 6.The semiconductor device of claim 3, wherein the at least one stressorlayer is a metal layer having a thickness of at least 50 μm, the metallayer arranged between the semiconductor semiconductor layer and anelectrically conducting carrier.
 7. The semiconductor device of claim 6,wherein the metal layer is a metal foil.
 8. The semiconductor device ofclaim 6, wherein the metal layer includes Cu.
 9. The semiconductordevice of claim 1, further comprising at least one stressor layerarranged at a front side of the vertical power semiconductor chipopposite a rear side, the vertical power semiconductor chip arranged onan electrically conducting carrier via the rear side.
 10. Thesemiconductor device of claim 9, wherein the at least one stressor layerincludes at least one of oxide, silicide, polysilicon, organic coating,metal, and metal foil.
 11. The semiconductor device of claim 9, whereinthe at least one stressor layer includes a metal layer having athickness of more than 10% of the thickness of the electricallyconducting carrier.
 12. The semiconductor device of claim 9, wherein theat least one stressor layer includes a metal layer having a thicknesslarger than an overall thickness of both the semiconductor layer and asemiconductor substrate adjoining the at least one stressor layer. 13.The semiconductor device of claim 9, wherein the at least one stressorlayer includes a metal layer having a thickness that is at least 20times larger than a thickness of a metal layer at the rear side of thepower semiconductor chip.
 14. The semiconductor device of claim 1,wherein the power semiconductor chip is arranged on a Direct CopperBonded (DCB) substrate.
 15. The semiconductor device of claim 1, whereina ratio of a thickness of the at least one stressor layer to thethickness of the semiconductor layer is between 2 and
 20. 16. Thesemiconductor device of claim 1, wherein the vertical powersemiconductor chip includes a vertical power semiconductor transistorhaving an operation voltage equal to or larger than 200 V.
 17. Avertical power semiconductor transistor, comprising: a semiconductorlayer; a first terminal at a first side of the semiconductor layer; asecond terminal at a second side of the semiconductor layer opposite thefirst side along a first direction; a drift zone within thesemiconductor layer between the first terminal and the second terminal,the drift zone having, in a central part, a compressive stress of atleast 100 MPa along a second direction perpendicular to the firstdirection, the central part extending from 40% to 60% of an overallextension of the drift zone along the first direction and into a depthof the semiconductor layer of at least 10 μm with respect to at leastone of the first side and the second side of the semiconductor layer; atleast one stressor layer arranged above at least one of the first sideand the second side of the semiconductor layer, the at least onestressor layer configured to induce the compressive stress within thedrift zone; and wherein the vertical power semiconductor transistor isconfigured to be operated at a voltage equal to or larger than 200 V.18. The vertical power semiconductor transistor of claim 17, wherein theat least one stressor layer adjoins the semiconductor layer.
 19. Thevertical power semiconductor transistor of claim 18, wherein the atleast one stressor layer is a metal layer having a thickness of at least50 μm, the metal layer arranged between the semiconductor layer and anelectrically conducting carrier.
 20. The vertical power semiconductortransistor of claim 19, wherein the metal layer is a metal foil.
 21. Thevertical power semiconductor transistor of claim 17, wherein the atleast one stressor layer is arranged at a front side of the verticalpower semiconductor transistor opposite a rear side, the vertical powersemiconductor transistor arranged on an electrically conducting carriervia the rear side.
 22. The vertical power semiconductor transistor ofclaim 17, wherein a ratio of a thickness of the at least one stressorlayer to the thickness of the semiconductor layer is between 2 and 20.23. The vertical power semiconductor transistor of claim 17, wherein thevertical power semiconductor transistor is arranged on a Direct CopperBonded (DCB) substrate.